Get 8*8 blocks of an image to an arrayList

Question

This is my code to take an ArrayList of 8*8 blocks of an image (Bmp) and it's not working. What is the flaw of the logic here? 'rows' is the number of rows of the image. I am doing my video steganography project.

        int x = 0;
        int colCount = 0;

        int yStart = 0;
        int yLimit = 8;

        for (x = 0;x < rows; x++)
        {
            while (yStart<yLimit)
            {
                imageBlock[x % 8, yStart % 8] = image_array[0].Data[x, yStart, 1];

                if (x % 8 == 7 && yStart % 8 == 7)
                {
                    blockList.Add(ForwardDCT(imageBlock));
                }

                yStart++;
            }

            if (x == rows - 1)
            { 
                x = 0;
                yLimit = yLimit + 8;
                //yStart = yStart + 8;
                colCount++;

                if (colCount == 100)
                {
                    break;
                }
            }

            if (yStart % 8 == 7)
            {
                yStart = yLimit - 8;
            }
        }

Answer 1

The flaw in the logic is that you've got a single imageBlock, and you overwrite the data in it several times before calling ForwardDCT(imageBlock).

You should change your loop structure to go in grid steps 8x8, then have two more loops that copy the content of the grid into imageBlock, and call ForwardDCT(imageBlock) on it rigth away:

for (var blockRow = 0 ; blockRow < rows ; blockRow += 8) {
    for (var blockCol = 0 ; blockCol < yLimit ; blockCol += 8) {
        for (var r = 0 ; r != 8 ; r++) {
            for (var c = 0 ; c != 8 ; c++) {
                // Add a check to see that blockRow+r, blockCol+c
                // are inside the original image
                imageBlock[r][c] = image_array[0].Data[blockRow+r, blockCol+c, 1];
            }
        }
        blockList.Add(ForwardDCT(imageBlock));
    }
}